The utilization of plastic waste is of considerable social significance. The present invention relates to a process for the maximal conversion of plastic waste into power and, hence, for the complete and ecologically safe disposal of unsorted, unpurified mixed plastic waste. The problem addressed by the invention was to eliminate the need for disposal at waste disposal sites, which is only possible to a limited extent, or for incineration and to enable mixed plastic waste of any origin, i.e. in the state in which it accumulates on collection, to be additionally utilized to the maximum extent in such a way that primary energy sources (such as gas, oil and coal) could be saved so that recycling, i.e. making hydrocarbons available, would be possible.
There are many known processes for recycling polymers and polymer degradation products which have to take the material composition of the waste plastic into consideration to varying degrees. Material utilization is only possible to a limited extent. The recycling processes are divided into physical/thermal, chemical/thermal and chemical processes.
The physical/thermal processes comprise melting and re-forming of the plastic waste. DE-PS 36 03 009 describes one such process. These processes generally involve the following disadvantages:
The polymer waste has to be purified and sorted; however, this is never the case in practice. PA1 Polymers age to a more or less considerable extent, so that their quality properties deteriorate appreciably after such recycling processes so that they can no longer be used for many applications. PA1 Polymers are only able to withstand a limited number of cycles in physical/thermal recycling processes, i.e. final disposal is still necessary. PA1 Due to the presence of chlorine (PVC), dioxins are always likely to be formed during the incineration process. At the same time, NO.sub.x is formed where incineration is carried out at high flame temperatures; the subsequent purification of waste gases is extremely expensive; PA1 Except for low-level steam generation, the plastic waste is not utilized, but merely disposed of. PA1 Due to the high degree of crosslinking of polymers, the chemical reactivity of the plastics during gasification under conventional process conditions is poor; PA1 The heat transport processes in the particles are a problem at the temperatures applied; PA1 Melting of the thermoplastics introduced into the gasification reactor leads to possible disturbances in the fluidized bed; PA1 The plastics are merely converted into lean gas of low calorific value during the gasification process; PA1 Through the use of oxygen, there is always a risk of dangerous organic chlorine compounds being formed. PA1 Pyrolysis (or gasification) of the plastic waste in a plasma reactor under reducing conditions at temperatures above 1200.degree. C. in the presence of substoichiometric oxygen: PA1 Cooling of the plasma by quenching and indirect heat transfer to produce steam; PA1 Cooling and purification of the plasma pyrolysis gas to remove heavy metals and traces of acids (for example H.sub.2 S, HCl, HF and HCN); PA1 Compression of the plasma pyrolysis gas to a pressure of &gt;10 bar; PA1 Heating of the plasma pyrolysis gas to a temperature near the temperature of the plasma pyrolysis gas before its purification; PA1 Combustion of the reheated plasma pyrolysis gas in the gas turbine to produce electricity with high exergetic efficiency; PA1 Passing the waste gases from the gas turbine into a boiler to generate steam; PA1 Using the exergetic steam potential in a following steam turbine to generate electricity.
At the present time, disposal at waste disposal sites is still the most common form of final disposal although in the future incineration is likely to be preferred.
The incineration of plastics as an often preferred end solution has the following disadvantages:
Further possibilities for the disposal of plastic waste lie in fluidized bed gasification. Although this variant would enable the plastic waste to be utilized as energy, it would entail the following disadvantages:
Substantially the same arguments may be applied to more recent gasification processes introduced into the debate (Bandermann, K., "Abfalle im Sauerstoffstrom", VDI-Nachrichten 6, Feb. 7, 1992, page 28; Menges G. and Fischer R., "Kohlenstoffrecycling beim Aufarbeiten gemischter Kunststoffabfalle (Recycling of Carbon in the Working up of Mixed Plastic Waste)", Kunststoffe 81 (1991)).
In addition to the gasification of plastics, there are a number of pyrolysis processes (see, for example, DE 33 23 161 and DE 35 31 514) which operate at low to medium temperatures (400.degree. to 900.degree. C.). These processes provide gaseous pyrolysis products and oils from the pyrolysis stage which are generally incinerated in a second high-temperature incineration stage. The gases and oils can be utilized to a limited extent for their energy content. However, chlorinated hydrocarbons (including above all chlorinated aromatic hydrocarbons) are formed to a large extent in the pyrolysis stage and represent a potential risk through the formation of dioxins and furans in the subsequent incineration process.
One feature common to all the various processes mentioned is that the plastics disposed of can only be utilized inefficiently, if at all, as an energy source although their energy content is considerably higher than that of lignite or mineral coal. Partial material utilization is only possible in exceptional cases. The products formed in conventional processes are generally difficult to market, although there is an almost unlimited market for power.